Available electrochromic devices may be divided into at least four classes:
Firstly, there are devices based on ion insertion reactions at essentially dense metal oxide electrodes. To ensure the desired change in transmittance needed to bring about a color change, a certain number of ions must be intercalated into the electrode to compensate for the accumulated charge. However, since the surface area in contact with the electrolyte is not significantly larger than the geometric area of the electrode, the use of metal oxide electrodes requires bulk intercalation of ions. As a consequence, the switching times of such a device are typically of the order of tens of seconds. The charge required for the coloration is counterbalanced by oxidation of a redoxcomponent, such as ferrocene or the like, in the electrolyte at the counter electrode, or by oxidation of a NiOxHy film on the counter electrode.
In U.S. Pat. No. 4,561,729 an electrochromic indicating device of this type is presented, which comprises a counter electrode comprised of activated carbon. This counter electrode counterbalances the charge required for the coloration of a coloring electrode of intercalation type by capacitive charging. In U.S. Pat. Nos. 5,940,202 and 5,708,523, similar electrochromic devices are shown, each comprising a porous carbon counter electrode, which is formed in a linear pattern and a dotted pattern, respectively.
Secondly, there are solution-phase electrochromic devices, in which one or two electrochromic compounds are dissolved in an electrolyte between two electrodes. Such systems are disclosed in U.S. Pat Nos. 4,902,108 and 5,128,799, wherein a molecule which colors by reduction and another one which colors by oxidation are present in the solution. The application of a voltage over the electrodes then results in the reduction of the former substance at the cathode and oxidation of the later at the anode. Systems of this type are not bistable and therefore bleach spontaneously when the current is off.
Thirdly, there are devices based on a transparent conducting substrate coated with a polymer to which a redox chromophore is bound. On applying a sufficiently negative potential there is a transmittance change due to formation of the reduced form of the redox chromophore. To ensure the desired change in transmittance a sufficiently thick polymer layer is required, the latter implying the absence of an intimate contact between the transparent conducting substrate and a significant fraction of the redox chromophores in the polymer film. As a consequence the switching times of such a device are, as for the first type discussed above, typically of the order of tens of seconds.
Fourthly, there are devices wherein at least one of the electrodes comprises a (semi-) conducting nanocrystalline film. The nanocrystalline coloring electrode is in this case formed of a metal oxide which carries a monolayer of adsorbed electrochromophoric molecules. These molecules comprise firstly an attachment group and secondly an electrochromophoric group, which do not absorb visible light in the oxidized state, but does absorb light in the reduced state (type n electrochromophore). More in detail, the extinction associated with the reduction of one type n electrochromophore is in the order of a magnitude higher than that of one electron in the nanostructured metal oxide. Further, due to the nanoporous structure and associated surface roughness of the nanocrystalline films used, the redox chromophore is effectively stacked as in a polymer film, while at the same time maintaining the intimate contact with the metal oxide substrate necessary to ensure rapid switching times. Alternatively, the electrochromophoric group may be a type p electrochromophore, whereby it exhibits a reverse behavior, i.e. it does absorb light in the oxidized state, but it does not absorb light in the reduced state. More generally, the electrochromophoric group may have two or more oxidation states with different colours. In such cases it is not possible to classify the electrochromophore as n-type or p-type.
A “nanocrystalline film” is constituted from fused nanometer-scale crystallites. In a “nanoporous-nanocrystalline” film the morphology of the fused nanocrystallites is such that it is porous on the nanometer-scale. The porosity of a nanostructured film is typically in the range of 50-60%, and the particle size is typically within the range of from a few nanometers up to several hundred nanometers in at least two dimensions (i.e the particles may be shaped as spheres, rods, cylinders e.t.c.). The thickness of a nanostructured film is typically in the order of 5-10 μm, but may be up to several hundred μm. Such films, which may hereinafter be referred to as nanostructured films, typically possess a surface roughness of about 1000 assuming a thickness of about 10 μm. Surface roughness is defined as the true internal surface area divided by the projected area.
The basic concept of nanostructured thin films is described by B. O'Regan and M. Grätzel in Nature, 353, 737 (1991), and by Grätzel et al in J. Am. Chem. Soc., 115, 6382 (1993).
The application of nanostructured thin films in electrochromic devices is described by A. Hagfeldt, L. Walder and M. Grätzel in Proc. Soc. Photo-Opt. Intrum. Engn., 2531, 60 (1995), and by P. Bonhôte, E. Gogniat, F. Campus, L. Walder and M. Grätzel in Displays 20 (1999) 137-144.
In their article Bonhôte et al disclose an electrochromic system wherein a nanostructured electrode with a type n electrochromophore added to the surface is used in conjunction with a metal counter electrode (e.g. Zn) and ions of the same metal in the electrolyte. The charge needed for coloration of the nanostructured electrode is achieved by oxidation of the metal resulting in dissolution of metal ions and subsequent bleaching by redeposition of the metal. Long-term stability appears to be poor as prolonged cycling leads to metal particle and dendrite formation. It is also known that such electrochromic devices exhibit problems with bubble formation. This may be solved by a special chemical treatment that also adds requirements on the composition of the electrolyte. The unstable metal surface is further not a good substrate for additional films, e.g. a white reflector. Metal ions may also be deposited in the coloring electrode or in other locations in the display, causing unwanted irreversible blackening.
In WO9735227 (U.S. Pat. No. 6,067,184) P. Bonhôte et al lay forward several electrode-combinations for electrochromic devices comprising one or two nanostructured electrodes with electrochromophores added to the surface. In a first embodiment a nanostructured electrode with a type n electrochromophore added to the surface, is used in conjunction with a transparent material of polymeric type, which is oxidizable, colorless in the reduced state and, respectively, colorless or colored in the oxidized state. In another embodiment, both electrodes are nanostructured and have adsorbed molecules on the surface, type n electrochromophoric on the cathode and type p electrochromophoric on the anode, whereby a device with two coloring electrodes is achieved. A third proposed embodiment comprises a nanostructured coloring-electrode (anode) with adsorbed molecules on the surface (type p electrochromophoric), and a nanostructured counter electrode with no adsorbed molecules. In this device, the charge required for the coloration is counterbalanced by insertion of small cations into the nanostructured counter electrode
In WO9835267 Fitzmaurice et al disclose that a nanostructured film, if used without the adsorbed redox chromophores, becomes colored on application of a potential sufficiently negative to accumulate electrons in the available trap and conduction band states. As a consequence of the high surface roughness of these films, ions are readily adsorbed/intercalated at the oxide surface permitting efficient charge compensation and rapid switching, i.e. the need for bulk intercalation is eliminated. However, Enright et al mentions that, despite the rapid switching times in such films, the associated change in transmittance is not sufficient for a commercial device. To overcome this limitation they propose that redox chromophores are adsorbed at the surface of the transparent nanostructured film, just as in the devices described above. This coloring electrode is then used in conjunction with an electrolyte containing ferrocene and a conducting glass counter electrode. Upon application of a voltage, reduction takes place at the coloring electrode and oxidation of ferrocene at the counter electrode. As reduced and oxidized species react internally, a permanent application of voltage is required to maintain the colored state i.e. there is no memory-effect. Naked electrodes, that is electrodes which does not have an redox active species added to the surface, have not been considered in previous displays with electrochromic capacitive electrodes, neither as non-colouring counter electrodes nor as counter electrodes, since naked electrodes has been considered to have too low charge capacity and colouration efficiency to be used in displays.